It is proposed to develop and validate a new assay system to replace existing immunocytochemical assays for detecting occult micrometastases in bone marrow of patients with primary breast cancer. The proposed assay would offer simplicity, low cost, and a quantitative end point. The assay is based on an innovative amplification mechanism, the bacterial chain reaction (BCR), which is potentially capable of lowering the detection limit to one micrometastasis in 10 million bone marrow cells. The BCR utilizes living cells of an autosyntrophic strain of Escherichia coil to amplify signals from reporter molecules, e.g., enzyme-labeled monoclonal antibodies. Under the BCR conditions, reporter molecules trigger nearby autosyntrophic cells to initiate a proliferative chain reaction resulting in discrete visible bacterial colonies clustered around tumor cells. The objectives in Phase I are to optimize the BCR assay and assess its detection limits of using a model system. For Phase II, the BCR assay will be validated by applying it to detection and enumeration of occult micrometastases in bone marrow specimens from patients with primary breast cancer. The final objective is to produce a BCR assay kit to be marketed as a diagnostic tool in the clinic.